There is a lot of recent scientific literature on metal chalcogenide nanoparticles with semiconductor properties. For instance, doped ZnS particles (ref. 1-9) can be used in eletroluminescent (EL) devices; CdS can be used in solar cells (ref. 10-14).
The precipitation of such metal chalcogenide nanoparticles can be performed homogeneously in water (ref. 1-6) or heterogeneously from micelles (ref. 7-9). After precipitation these particles are filtrated, washed and dried, after which their fotoluminescence or electroluminescence properties can be studied. The drawback of this method is that due to these filtration, wash and drying step the particles will inevitably tend to agglomerate. This is disadvantageous for their physico-chemical properties and possibly also for their luminescence properties. Centrifugation and flocculation show the same drawbacks.
With ZnS dispersion there is following additional problem: by hydrolysis/oxidation of the ZnS particle surface hydroxide-like gels arise which during concentration steps inevitably give rise to gelation and agglomeration
In order to be able to coat functionally useful layers of nanoparticles (average particle size<100 nm) it is necessary to concentrate the coating dispersion sufficiently. By doing so it becomes possible to coat enough active compound per m2 with conventional coating techniques. For example, for a coverage of 2 g Zn(Mn)S per m2 and a wet coating thickness of 40 μm a concentration of 50 g Zn(Mn)S nanoparticles per liter is needed.
So there is a need for an improved method for washing and concentrating of precipitated metal chalcogenide particles without the occurence of agglomeration.
Ultrafiltration and diafiltration are well-known methods for washing and concentrating precipitated metal salts. However when applying this to ZnS gelation occurs and due to the increasing viscosity the ultrafiltration membrane will clog.
The use of surface-active compounds capable of stopping the growth of chalcogenides are known from literature. They are used to stop the growth of very fine nanoparicles with average size below 10 nm. Most commonly mentioned is thioglycerol (ref. 9, 13). However when using this compound during precipitation the growth of the particles is disturbed. In this case the particles remain so small that the dispersion cannot be ultrafiltrated.
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